The present invention relates, in general, to systems containing a sensing component and, more particularly, to sensing structures comprising a moveable component such as found in a transducer.
Capacitive transducers are one type of micromachined sensors using a moveable component to detect acceleration. Capacitive transducers are used in automotive applications as accelerometers in crash sensing for air-bag deployment. Other applications include ride control, inertial navigation, and virtual reality systems. The moveable component in the sensor forms one plate of a capacitor and a variation in spacing between the moveable component and a substantially stationary conductive plate produces a corresponding variation in capacitance value. A sensing circuit coupled to the capacitor detects the change in capacitance value and provides a measurement of the force that caused movement of the moveable component.
The capacitor structure comprises three polysilicon layers that form two capacitors. The first capacitor is formed by a bottom polysilicon layer that adheres to the substrate and a middle polysilicon layer that is suspended by support beams. The second capacitor is formed by the middle polysilicon layer and a top polysilicon layer that is also anchored at various points to the substrate. Mechanically, the bottom and the top polysilicon layers have little motion relative to the substrate and are considered as rigid bodies. The middle layer is deflected either toward the top polysilicon layer or toward the bottom polysilicon layer when subjected to a force with a component in the compliant direction, resulting in changes in capacitance for both capacitors.
Support beams provide support for the middle polysilicon layer and can be made of a thin layer of silicon nitride sandwiched between two polysilicon layers. Although the doped polysilicon can have a compressive force, the combination of the silicon nitride and polysilicon layers provides a tensile force capable of suspending the middle polysilicon layer about equal distances between the top and bottom polysilicon layers. However, the difference in temperature coefficients between the silicon nitride and the doped polysilicon in the support beams causes the tension in the beams to vary as the temperature varies, resulting in a change of the overall structural stiffness and sensitivity of the sensor.
Accordingly, there is a need for a capacitor sensor having a moveable component to detect acceleration where the support beams are elastic and provide stable mechanical support. It would be a further advantage to have a sensor that is easily and inexpensively manufactured, minimizes the effects of temperature variations on the sensitivity of the sensor, and is insensitive to residual film stress and externally imposed stress.